1. Field of the Invention
The invention relates to a metering oven with a vessel for receiving liquid metal and a device for detecting a liquid metal level in a vessel.
2. Description of the Prior Art
For metering liquid metal from a metering oven, the height of the column of metal rising in the metering tube must be detected, since the metering amount is calculated on the basis of this detection. It is also possible, in dependence on the detection of the height of the metal column, to determine the height of the liquid level in the oven taking into account other parameters, for example different pressures. From U.S. Pat. No. 4,220,319 a sensor arrangement for metering ovens is known in which the sensor consists of a metal needle standing perpendicular or almost perpendicular to the metal surface, and which emits a signal on contact with the surface of the liquid metal. In order to reduce the wear on the sensor arrangement, the metal needle is swung away from the metal surface by an automatic mechanical system on contact. This known arrangement has various disadvantages; in particular the mechanical swivel system involves high outlay and is very expensive and, in spite of the swivelling, the wear on the metal needle is relatively high. The needle can first of all be decomposed by the contact with liquid aluminium on the basis of chemical processes. Moreover, the measuring result can be impaired by the accumulation of aluminium or aluminium oxide on the needle.
In practice, because of the above-explained wear of the metal needle, grinding, cleaning or exchange of the needle is necessary, such that the scanning position cannot be held over a lengthy period of time. Furthermore, there are no adjustment aids known in practice which make reproducible scanning possible. In particular, the scanning position in relation to the discharge edge of the metering tube is of particular importance in metering ovens. For metering which may be reproduced very well, the needle should detect exactly the discharge position of the liquid metal at the discharge edge of the metering tube (needle and discharge edge must sit at the same height). The detection, however is displaced in practice not only by the above-mentioned maintenance and repair work on the metal needle, but also through the exchange of the metering tube, whose installation height directly determines the position of the discharge edge. Determined by large production-engineering tolerances in the refractory sphere, the installation of a new metering tube as well as a new seal etc. can displace the discharge edge by up to 10 mm in vertical height.
On metering ovens, a displacement of the scanning position in relation to the discharge edge, as a result of the above-mentioned measures, of for example 5 mm causes an alteration in the metered metal weight of typically 4%. A metering accuracy of 1 to 2% is required. Because of the bad access and the heat which prevails in the scanning region, in practice the needle is not subsequently adjusted; instead, the pressure or time parameters of the metering, or in our case the metering weight, which is determined in known manner according to the integral method (pressure over time), are changed in order to compensate for the distortion in the metering weight. This has the disadvantage that founders who have stored the metering parameters of various castings have to undertake corrections to these stored values again and again, since the scanning conditions and thus the metering do not in fact remain constant.
A further disadvantage of the metal needle portrayed above arises from the underlying principle (on account of the necessary contact with liquid molten metal). A layer forming after the shortest time on the surface of the molten metal, for example of non-conductive aluminium oxide, must first be broken through by the needle. As a result of the pressure of the needle, bulging of the metal surface downwards occurs. This causes, on the one hand, an inaccurate measurement result (the needle, after breaking through in a position which is too deep, emits its signal, i.e. less molten metal is indicated than is actually present). Moreover, after the oxide surface has been broken through, there is unnecessarily deep plunging of the needle into the liquid molten metal, such that the wear of the metal needle described above is accelerated.
From DE-OS 44 20 712 is known, furthermore, a sensor arrangement for detecting the level of liquid metal, in which arrangement the sensor consists of electrically-conductive ceramics and is inserted flush into the wall of the vessel or of an ascending pipe.
JP-A-05099726 is regarded as the definitive state of the art. It discloses a pipe which is disposed in the wall of the tundish filled with molten metal and is open towards the interior of the tundish, the pipe opening being disposed below the upper edge of the liquid molten metal. Gas is blown out of the pipe into the molten metal. The counter pressure generated in the pipe by the molten metal (trhough which pressure the filling height of the molten metal can be inferred) is detected by means of a pressure measuring device.
To measure an exact filling height, which would meet the requirements of a metering oven, an expensive pressure measuring device would be needed here however, and this device would, moreover, need to be constantly calibrated involving high outlay.
The problem underlying the invention is to create a metering oven with a vessel for receiving liquid metal and a device for detecting a level of liquid metal in a vessel, in which the device detects the level with high accuracy.